4-tert-Octylphenol is toxic even at nanomolar levels but widely used as an intermediate substance. The detection methods for 4-tert-octylphenol at nanomolar levels are however missing in literature. We investigate in this paper the voltammetric response and electrochemical detection of nanomolar leveled 4-tert-octylphenol in water. A polymer/CNT film coated electrode was applied. The electrode was prepared by firstly coating a polished and clean glassy carbon electrode with a CNT film and then covering the CNT film coated electrode with a conducting film from L-lysine. On this electrode, 4-tert-octylphenol is electrochemically oxidized at 0.60 V vs. SCE. This irreversible oxidation process is controlled by adsorption. The anodic peak current is proportional to the concentration of 4-tert-octlyphenol in the range of 6.5-20 nM. The detection limit is 0.5 nM. This electrode has been applied for the detection of 4-tert-octylphenol in lake and river water. The relative standard derivation (RSD) of these measurements is less than 4.3%. (C) 2012 Elsevier B.V. All rights reserved.